This invention relates to the manufacture of exterior automotive body panels or members made from polymeric materials, and more particularly, to a dry paint transfer-lamination process for applying preformed paint film laminates to plastic substrate sheet materials. The process avoids the separate step of injection molding a substrate panel to the dry paint transfer laminate.
Exterior automotive body panels have been made in the past by spray painting sheet metal parts. Multi-layer paint coats, such as those referred to as clear coat/color coat paint systems, have been used to produce desirable optical effects. In addition to high gloss and high distinctness-of-image (DOI), these paint coats also are highly durable by providing chemical resistance, abrasion resistance and weatherability that avoids degradation by ultraviolet light.
In more recent years, molded plastic car body panels have been made with decorative clear coat/color coat paint films bonded to the molded plastic panel. Use of such films avoids certain environmental problems associated with evaporation of paint solvents while also reducing or eliminating the need for paint facilities and emission controls at the automotive production plant.
Because of the growing need to reduce atmospheric pollution caused by solvents emitted during the painting process, many different approaches have been taken in recent years for producing these decorative films. These processes can be generally categorized by solution-casting techniques or extrusion techniques. For instance, U.S. Pat. No. 5,707,697 to Spain et al. describes solution-casting techniques in which liquid-cast solvent-based clear coats and pigmented base coats are applied to a flexible casting sheet by a coating process such as reverse roll coating. The liquid-cast layers are separately applied and then dried at high temperatures to evaporate the solvents. Following the steps of casting the clear coat and color coat, the composite paint coat is removed from the carrier and transfer laminated to a thin, semi-rigid, thermoformable polymeric backing sheet. The preferred backing sheet is about 20 mils in thickness, although the backing sheet can have a thickness from about 10 mils to about 40 mils. The paint coated backing sheet is then thermoformed into a desired three-dimensional shape, followed by molding the substrate panel to the thermoformed sheet in an injection mold. This process, known as the xe2x80x9cinsert-moldxe2x80x9d process, together with the transfer-lamination and thermoforming steps, are described in the above-mentioned ""697 patent which is incorporated herein by this reference.
As an alternative to solvent-cast films, extruded films have been used for making exterior automotive clear coat/color coat paint films. International Application WO 96/40480 to Enlow et al., which is incorporated herein by this reference, describes an approach in which paint films and/or laminates are made by extrusion coating or coextrusion techniques. The paint film is transferred to a supportive backing sheet by transfer-lamination techniques, followed by thermoforming the paint coated backing sheet into a desired three-dimensional shape and injection cladding it to a substrate panel by the insert-mold process.
As a further alternative, a dry paint transfer sheet can be placed directly in an injection mold without thermoforming it outside the mold. The sheet in this instance is formed into a contoured shape under heat and pressure in the injection mold by the molding material in what is referred to as the xe2x80x9cin-moldxe2x80x9d process. Such a process, for example, is described in U.S. Pat. No. 4,810,540 to Ellison et al.
The present invention is based on a recognition that production costs for making these exterior automotive parts can be reduced by eliminating the injection-molding step. In other words, it can be desirable to manufacture an exterior automotive body member or panel so that the finished automotive part, such as a facia, is ready to put on the vehicle after the thermoforming step. This would require producing a polymeric substrate sheet with sufficient thickness and structural integrity to function as the finished part that adequately supports the transferred paint film. This also requires a polymeric substrate material that can be shaped by thermoforming techniques while not interfering with the desired optical properties (such as high distinctness-of-image and gloss) of the finished paint film. For instance, may be desirable to use polymeric substrate materials that contain high filler or regrind components in order to reduce the cost of the overall product. But these particulate materials can be transmitted from the substrate to the paint film during thermoforming, creating imperfections that degrade the otherwise desired smooth, high quality optical surface of the finished film. In addition, the thicker substrate sheet material can absorb substantial amounts of heat when bonding a paint film to it or when thermoforming it. Such excessive heat absorption can be transferred to the paint film surface and degrade the optical properties of the paint film by causing excessive haze or fogging of the clear coat. This problem is particularly critical when such exterior paint films are made from fluoropolymer resins, such as polyvinylidene fluoride (PVDF) and acrylic resin alloys of the types described in the above-mentioned ""697 patent to Spain et al., International Application WO 96-40480 to Enlow et al., or the ""540 patent to Ellison et al.
Briefly, one embodiment of this invention provides a process for making exterior automotive quality body members or panels from a preformed dry paint transfer laminate comprising an exterior automotive quality paint film applied to a relatively thin, semi-rigid, thermoformable polymeric backing sheet. The process includes the step of extruding a relatively thick sheet of a thermoformable polymeric material and forming a paint coated thick sheet laminate by laminating the backing sheet side of the preformed dry paint laminate to the extruded sheet. The lamination step is carried out using the heat of extrusion to heat bond the backing sheet and extruded sheet together to form an integral substrate sheet which is thermoformable to a three-dimensional shape with the dry paint film bonded to its exterior surface. The backing sheet and extruded sheet comprise compatible polymeric materials to promote the bond between them. In one embodiment the extruded sheet has a thickness of at least twice the thickness of the backing sheet. In another embodiment the backing sheet has a thickness from about 10 mils to about 30 mils, and the extruded substrate sheet has a thickness of at least about 40 mils. In forming the thick sheet laminate to a desired shape, heat is initially applied to the laminate to gradually raise the substrate temperature to the forming temperature. This step is followed by forming the laminate to a three-dimensional shape, preferably by vacuum forming, to produce a finished exterior automotive quality body panel or member.
The present invention provides a process for manufacturing a high quality exterior automotive paint finish (high gloss and high DOI) on a body member of panel of a thickness up to about 0.250 inch, by first laminating the dry paint transfer film to the relatively thin polymeric backing sheet, and subsequently laminating the backing sheet to the substantially thicker extruded substrate sheet. The intermediate backing sheet provides a barrier that prevents transmission of defects from filler and regrind materials contained in the substrate sheet material to the paint film surface, thereby preserving the high quality optical properties of the finished film during thermoforming to a finished three-dimensional shape.
A primary advantage of the process is the elimination of a subsequent injection molding step. This greatly reduces production costs because the high cost of an injection molding tool is avoided. Processing time also is reduced.
As mentioned, the paint film-backing sheet laminate is heat-bonded to the extruded substrate sheet, using the heat of extrusion to bond the backing sheet to the extruded substrate. This lowers production costs by reducing the overall number of steps in the process, when compared with laminating the backing sheet to a preformed substrate sheet or molded sheet.
In one form of the invention, the extruded substrate sheet is rapidly cooled following extrusion and prior to lamination to the dry paint transfer laminate. This avoids high levels of heat being transmitted to the paint film which could otherwise be at a level sufficient to cause haze or other degradation of the optical properties of the paint film. Haze is undesirable not only in the clear coat (because it lowers gloss and DOI) but also in the color coat, because it can produce undesired color changes causing difficulties in color matching. Further, preferred thermoforming techniques include preheating the extruded substrate sheet under controlled heating conditions prior to thermoforming. This can reduce the amount of heat to which the paint film is exposed during thermoforming to further control excessive heat transmission to the paint film.